The dipole moment of a system of charge ` +q` distribyted uniformly on an arc of radius `R` subtending an angle ` pi //2` at its centre where another charge ` -q` is placed is :
A. `(2sqrt(2)qR)/(pi)`
B. `(sqrt(2)qR)/(pi)`
C. `(qR)/(pi)`
D. `(2qR)/(pi)`